Flow field plate geometries

ABSTRACT

A flow field plate for a fuel cell or electmolyser comprises on at least one face an assembly of channels comprising one or more gas delivery channels, and a plurality of gas diffusion channels of width less than 0.2 mm connecting thereto.

[0001] This invention relates to fuel cells and electrolysers, and isparticularly, although not exclusively, applicable to proton exchangemembrane fuel cells and electrolysers.

[0002] Fuel cells are devices in which a fuel and an oxidant combine ina controlled manner to produce electricity directly. By directlyproducing electricity without intermediate combustion and generationsteps, the electrical efficiency of a fuel cell is higher than using thefuel in a traditional generator. This much is widely known. A fuel cellsounds simple and desirable but many man-years of work have beenexpended in recent years attempting to produce practical fuel cellsystems. An electrolyser is effectively a fuel cell in reverse, in whichelectricity is used to split water into hydrogen and oxygen.

[0003] Both fuel cells and electrolysers are likely to become importantparts of the so-called “hydrogen economy”. In the following, referenceis made to fuel cells, but it should be remembered that the sameprinciples apply to electrolysers. One type of fuel cell in commercialproduction is the so-called proton exchange membrane (PEM) fuel cell[sometimes called polymer electrolyte or solid polymer fuel cells(PEFCs)]. Such cells use hydrogen as a fuel and comprise an electricallyinsulating (but ionically conducting) polymer membrane having porouselectrodes disposed on both faces. The membrane is typically afluorosulphonate polymer and the electrodes typically comprise a noblemetal catalyst dispersed on a carbonaceous powder substrate. Thisassembly of electrodes and membrane is often referred to as the membraneelectrode assembly (MEA).

[0004] Hydrogen fuel is supplied to one electrode (the anode) where itis oxidised to release electrons to the anode and hydrogen ions to theelectrolyte. Oxidant (typically air or oxygen) is supplied to the otherelectrode (the cathode) where electrons from the cathode combine withthe oxygen and the hydrogen ions to produce water. A sub-class of protonexchange membrane fuel cell is the direct methanol fuel cell in whichmethanol is supplied as the fuel. This invention is intended to coversuch fuel cells and indeed any other fuel cell using a proton exchangemembrane.

[0005] In commercial PEM fuel cells many such membranes are stackedtogether separated by flow field plates (also referred to as bipolarplates). The flow field plates are typically formed of metal or graphiteto permit good transfer of electrons between the anode of one membraneand the cathode of the adjacent membrane. The flow field plates have apattern of grooves on their surface to supply fluid (fuel or oxidant)and to remove water produced as a reaction product of the fuel cell.

[0006] Various methods of producing the grooves have been described, forexample it has been proposed to form such grooves by machining,embossing or moulding (WO00/41260), and (as is particularly useful forthe present invention) by sandblasting through a resist (WO01/04982).

[0007] International patent application No. WO01/04982 disclosed amethod of machining flow field plates by means of applying a resist ormask to a plate and then using sandblasting (or other etching methodusing the momentum of moving particles to abrade the surface, e.g.waterjet machining), to form features corresponding to a pattern formedin the mask or resist.

[0008] Such a process was shown by WO01/04982 as capable of formingeither holes through the flow field plates, or closed bottom pits orchannels in the flow field plates. The process of WO01/04982 isincorporated herein in its entirety, as giving sufficient background toenable the invention.

[0009] In practice, the majority of plates made to date have been formedby milling the channels.

[0010] WO00/41260 discloses a flow field geometry in which substantiallystraight parallel channels are provided of a width less than about 0.75mm.

[0011] WO00/26981 discloses a similar geometry in which highly parallelflow channels of a width less than 800 μm separated by lands of lessthan 800 μm are used. This geometry is stated to improve gasdistribution as reducing the need for lateral gas dispersion through theMEA (referred to in WO00/26981 as the DCC [diffusion currentcollectors]). The geometry is also stated to reduce electricalresistance as it reduces the electrical path length to land areas. Thereis a conflict between electrical and gas properties described inWO00/26981, in that reduced land areas are stated to increase electricalresistance. WO00/26981 states that these conflicting requirements may beoptimised. WO00/26981 states that the pattern of highly parallelmicro-channels may contain interconnections or branch points such as inhatchings or grid patterns. One advantage of the use of narrow channelsis stated to be that this encourages water droplet formation across thechannels so permitting efficient water removal. However this advantagemay not be seen where a grid pattern is used as the pressure either sideof a water droplet is likely to be substantially equal.

[0012] Cited against WO00/26981 are:

[0013] U.S. Pat. No. 3,814,631, which discloses an electrodeconstruction in which micro-channels of more than 0.3 mm wide areprovided in a frame edge leading to a textured electrode in whichprotrusions on one face of the electrode match depressions in theopposed face of the electrode.

[0014] U.S. Pat. No. 5,108,849, which discloses a plate havingserpentine tracks of 0.76 mm (0.03 inch) width or more with land widthsof 0.254 mm (0.01 inch) or more.

[0015] WO94/11912, which discloses a plate having discontinuous tracksof 0.76 mm (0.03 inch) width and depth. These tracks may beinterdigitated.

[0016] WO98/52242, which discloses means for humidifying the membrane,

[0017] Narrow channels are known for other devices, for example,WO94/21372 discloses a chemical processing apparatus comprising a threedimensional tortuous channel formed by aligning part channels inadjacent discs. Such a construction has not been used for a fuel cell.

[0018] None of the fuel cell related patents disclose a structure ofcoarse gas delivery channels leading to fine gas diffusion channels.

[0019] To ensure that the fluids are dispersed evenly to theirrespective electrode surfaces a so-called gas diffusion layer (GDL) isplaced between the electrode and the flow field plate. The gas diffusionlayer is a porous material and typically comprises a carbon paper orcloth, often having a bonded layer of carbon powder on one face andcoated with a hydrophobic material to promote water rejection. It hasbeen proposed to provide an interdigitated flow field below amacroporous material (U.S. Pat. No. 5,641,586) having connected porosityof pore size range 20-100 μm allowing a reduction in size of the gasdiffusion layer. Such an arrangement permits gas flow around blockedpores, which is disadvantageous. Build up of reactant products (such aswater) can occur in these pores reducing gas transport efficiency.Additionally, such a structure increases the thickness of the flow fieldplate.

[0020] The inventors have analysed what happens in a fuel cell and havecome to the conclusion that the gas diffusion layer does not do whatit's name implies. The theory had been that the gas diffusion layerserves to permit gas to diffuse across the whole surface of the membraneso that large portions of the membrane are active in the cell reaction.The inventors have found that, in simple models, the gas appears not toaccess the whole of the lands between the channels, but only the areaabove the channels and a small margin surrounding the channels, themajority of the electricity generation taking place in this restrictedregion. This is supported by the observation that interdigitatedchannels show higher electrical efficiencies since the gas is forcedinto the areas above the lands. The gas diffusion layer does howeveractually serve a useful purpose in carrying current from those areas ofthe membrane electrode above the channels to the lands, and in providingmechanical support to the membrane electrode to prevent it beingsqueezed into the channels. Several inventors have proposed stiffeningthe membrane electrode.

[0021] The gas diffusion layer in carrying current from those areaswhere electricity is generated to the lands, does of course result inelectrical losses due to the electrical resistance of the gas diffusionlayer. Present day gas diffusion layers are chosen as a delicate balancebetween the needs of mechanical strength, electrical conductivity, andgas permeability.

[0022] A combined flow field plate and gas diffusion layer has beendescribed in U.S. Pat. No. 6,037,073 and comprises a selectivelyimpregnated body of porous carbon material, the impregnationhermetically sealing part of the plate. Such an arrangement has thedrawbacks that it is complicated to make reproducibly and that itpermits gas flow around blockages as in An assembled body of flow fieldplates and membranes with associated fuel and oxidant supply manifoldsis often referred to a fuel cell stack.

[0023] Although the technology described above has proved useful inprototype and in some limited commercial applications, to achieve widercommercial acceptance there is now a demand to reduce the physical sizeof a fuel cell stack and to reduce its cost. Accordingly, a reduction inthe number of components could have beneficial results on size and cost(both through material and assembly costs).

[0024] Also, the prior art flow field plates have provided flow fieldsof serpentine, linear, or interdigitated form but have not looked toother physical systems for improving the gas flow pathways. Suchexisting flow field patterns tend to have a problem with gas “shortcircuiting” by passing from one channel to an adjacent channel having asignificantly lower pressure.

[0025] The inventors have realised that by forming sufficiently finechannels on the face of the flow field plates the purpose ofdistributing the gas evenly across the electrodes can be achievedwithout the use of a separate gas diffusion layer. The membrane can beprevented from falling into the channels by use of a stiffened membraneor lower clamping pressures as appropriate.

[0026] The inventors have further realised that by looking tophysiological systems (the lung) improved flow field geometries may berealised that are likely to have lower parasitic losses due to theirshorter gas flow pathways. They have also realised that such geometriesare less likely to suffer from gas short-circuiting.

[0027] Additionally, the inventors have realised that use of narrowtracks results in a reduction in resistive electrical losses in the gasdiffusion layer, since there will be shorter pathways from theelectrically active regions of the membrane electrode to the landsbetween the channels. Conversely, as the pathways from the electricallyactive regions of the membrane electrode to the lands between thechannels are shorter, then a higher resistance gas diffusion layer canbe tolerated so permitting a wider range of materials to be consideredfor the gas diffusion layer.

[0028] The present invention therefore provides a flow field plate for afuel cell comprising on at least one face an assembly of channelscomprising one or more gas delivery channels, and a plurality of gasdiffusion channels of width less than 0.2 mm connecting thereto.

[0029] The gas delivery channels may comprise one or more primarychannels of a width greater than 1 mm, and a plurality of secondary gasdelivery channels of a width less than 1 mm connecting thereto.

[0030] The gas diffusion channels may form a branched structure.

[0031] The gas diffusion channels may be of varying width, forming abranched structure of progressively diminishing channel width similar tothe branching structure of blood vessels and air channels in the lung.

[0032] The invention is illustrated by way of non-limitative example inthe following description with reference to the drawing in which:

[0033]FIG. 1 shows schematically in part section a part of a fluid flowplate incorporating gas delivery channels and gas diffusion channelsformed by sandblasting;

[0034]FIG. 2 shows schematically a partial plan view of a fluid flowplate incorporating gas delivery channels and gas diffusion channels;

[0035]FIG. 3 shows schematically a prior art design flow field plate toillustrate the problem of short circuiting.

[0036]FIG. 4 shows schematically a prior art arrangement of channels;and

[0037]FIG. 5 shows a part section of a branched flow field pattern inaccordance with the present invention.

[0038] To form both gas delivery and gas diffusion channels a techniquesuch as sand blasting may be used in which a template or resist isplaced against the surface of a plate, the template or resist having apattern corresponding to the desired channel geometry. Such a techniqueis described in WO01/04982, which is incorporated herein in its entiretyas enabling the present invention. With this technique the plates may beformed from a graphite/resin composite or other non-porous electricallyconductive material that does not react significantly with the reactantsused.

[0039] It is found with this technique that the profiles of channels ofdifferent width vary due to the shadow cast by the mask. FIG. 1 shows aflow field plate 1 having a narrow channel 2 formed in its surface.Because of the shadowing effect of the resist used in forming thechannel the channel is exposed to sandblast grit coming effectively onlyfrom directly above. This leads to a generally semicircular profile tothe channel and to a shallow cutting of the channel.

[0040] For progressively larger channels (3 and 4) the resist casts lessof a shadow allowing sandblasting grit from a progressively wider rangeof angles to strike the surface of the flow field plate, so allowingboth deeper cutting of the surface and a progressively flatter bottom tothe channel.

[0041] Accordingly, by applying a resist with different width channelsto a plate and exposing the plate and resist to sandblasting with a finegrit, a pattern of channels of different widths and depths can beapplied.

[0042] Applying such a pattern of channels of varying width and depthhas advantages. In flow field plates the purpose behind the channelsconventionally applied is to try to ensure a uniform supply of reactantmaterial to the electrodes and to ensure prompt removal of reactedproducts. However the length of the passage material has to travel ishigh since a convoluted path is generally used.

[0043] Another system in which the aim is to supply reactant uniformlyto a reactant surface and to remove reacted products is the lung. In thelung an arrangement of progressively finer channels is provided so thatair has a short pathway to its reactant site in the lung, and carbondioxide has a short pathway out again. By providing a network ofprogressively finer channels into the flow field plate, reactant gaseshave a short pathway to their reactant sites.

[0044] The finest channels could simply discharge into wide gas removalchannels or, as in the lung, a corresponding network of progressivelywider channels could be provided out of the flow field plate. In thelatter case, the two networks of progressively finer channels andprogressively wider channels could be connected end-to-end or arrangedas interdigitated networks with diffusion through a gas diffusion layeror through the electrode material providing connectivity. Connectionend-to-end provides the advantage that a high pressure will bemaintained through the channels, assisting in the removal of blockages.

[0045]FIG. 2 shows in a schematic plan a portion of a flow field platehaving broad primary gas delivery channels 4, which diverge intosecondary gas delivery channels 3 which themselves diverge into gasdiffusion channels 2. Gas diffusion channels 5 can also come off theprimary gas delivery channels 4 if required. The primary and secondarygas delivery channels may each form a network of progressively finerchannels as may the gas diffusion channels and the arrangement of thechannels may resemble a fractal arrangement.

[0046] The primary gas delivery channels may have a width of greaterthan 1 mm, for example about 2 mm. The depth of such a channel islimited only by the need to have sufficient strength in the flow fieldplate after forming the channel. A typical channel is about 40% of theplate thickness. On current plates (6 mm thick) the channels aretypically 2.5 mm deep. As the plate becomes thinner then the channeldepth will reduce. However, the catalyst and GDL are soft materials thatwill intrude into a channel with a low aspect ratio (shallow and wide).Preferably therefore the aspect ratio of the channels is typicallybetween 0.5 and 2. The secondary gas delivery channels may have a widthof less than 1 mm, for example 0.5 mm and may be shallower than theprimary gas delivery channels. The gas diffusion channels have a widthof less than 0.2 mm, for example about 100 μm and may be shallowerstill.

[0047] By providing such a structure, reactant products have a shortdistance to travel and can be removed efficiently in comparison withconventional plate designs. Additionally, gas channels in typicalbipolar plates are of square or rectangular section and are millimetricin size. E.g. Ballard™ plates have a 2.5 mm square section channel. APS™plates have a channel that is 0.9 mm wide by 0.6 mm deep. Smallerchannels are beneficial as the pressure drop per unit length is higherand the pressure drop is what drives the reactants into the diffusionmedia.

[0048] In a conventional flow field plate 6, as shown in FIG. 3, gas(fuel or oxidant) enters a first port 7 and exits a second port 8. Thegas flows in a serpentine channel 9 from port 7 to port 8 diminishing inpressure as it does so. It will be appreciated that at certain parts 10of the track the pressure differential between adjacent tracks is highand this can result in gas short-circuiting the channel so that otherparts 11 are starved of fuel or oxidant. This short circuiting occurs bythe gas passing between the membrane electrode and the face of the flowfield plate. Most current plates have a pressure differential (ΔP)between inlet and outlet of <100 mbar on the air side. Interdigitatedplates have high ΔP, typically about 3 times atmospheric pressure.

[0049] In contrast, in the present invention, adjacent tracks may bedesigned to have broadly similar pressures, so reducing the risk ofshort-circuiting.

[0050] WO00/41260 has an extensive discussion of flow field design buthas not appreciated that be providing extremely fine channels (less than0.2 mm) and by providing such channels as part of a network ofprogressively diminishing width, the pressure drop between adjacentchannels is minimised so avoiding short-circuiting of the flow field.

[0051] The primary channel(s) must be of a size sufficient to deliverthe working volume of gas required by the cell. This is about 25 L/minper kW of working power.

[0052] The flow field plates may be used with a gas diffusion layer, orthe gas diffusion channels may be provided in a sufficient density overthe surface of the flow field plate to provide sufficient gas deliverythat a gas diffusion layer may be omitted, or significantly reduced inthickness. Such reduction is considered advantageous as the GDLcomponent is a major contributor to resistive losses in the cell.

[0053] The limit on channel width is a function of the mask thicknessused in the sand blast process. Image Pro™ materials (Chromaline Corp.US), are very thick at 125 micron. These masks limit track width toabout 100 microns. Other mask materials can be spray coated onto thesubstrate and exposed in situ. These materials are much more resilientand hence can be much thinner. Chromaline SBX™ can be used to etchfeatures down to 10-20 microns wide.

[0054] WO0/26981 discusses the use of flow fields having parallelchannels which may have branching or interconnecting points such ashatchings or grid patterns. Such a pattern has significant drawbacks andis shown schematically in FIG. 4. As indicated in FIG. 4, a grid ofchannels 12 is provided. If a droplet of water 13 blocks one of thechannels, reactant gas can easily flow around the droplet followingarrow 14. This will result in the pressure downstream (A) of the dropletbeing very close to the pressure upstream (B) and so there will belittle driving force to remove the droplet.

[0055] In contrast, with a branched flow field as shown in FIG. 5, gasflows in a branching pattern 15 the pathway for reactant gas from theupstream side of droplet 13 to the downstream side of droplet 13 islong—effectively to the end of the flow field and back again. This meansthat the pressure upstream (B) of the droplet will be significantlyhigher than the pressure downstream (A), so providing a driving forcefor removal of water.

[0056] As well known, (see for example WO00/41260) the same pattern ofgrooves does not need to be applied to both faces of a flow field plateand the present invention is not limited in this way.

[0057] It is known to provide flow field plates comprising anelectrically conductive core and a non-conductive frame (e.g.WO97/50139, WO01/89019, and U.S. Pat. No. 3,278,336). The flow fields ofthe present invention may be used in such arrangements, with either theentire flow field being on the conductive core, or being partially onthe non-conductive frame and partially on the conductive core.

1. A fuel cell flow field plate comprising on at least one face anassembly of channels comprising one or more gas delivery channels, and aplurality of gas diffusion channels of width less than 0.2 mm connectingthereto.
 2. An electrolyser flow field plate comprising on at least oneface an assembly of channels comprising one or more gas deliverychannels, and a plurality of gas diffusion channels of width less than0.2 mm connecting thereto.
 3. A flow field plate as claimed in claim 1or claim 2, in which the gas delivery channels comprise one or moreprimary channels of a width greater than 1 mm, and a plurality ofsecondary gas delivery channels of a width less than 1 mm connectingthereto
 4. A flow field plate as claimed in any of claims 1 to 3, inwhich the gas diffusion channels form a branched structure.
 5. A flowfield plate as claimed in claim 4 in which the gas diffusion channelsare of varying width forming a branched structure of progressivelydiminishing channel width.
 6. A flow field plate as claimed in anypreceding claim comprising a first assembly of channels for gas deliveryand a second assembly of channels for removal of reactant products.
 7. Aflow field plate as claimed in claim 6, in which the first and secondassemblies of channels are interdigitated.
 8. A flow field plate asclaimed in any preceding claim in which channels decrease in depth withdiminishing width.
 9. A flow field plate as claimed in any precedingclaim in which the gas diffusion channels are provided in a sufficientdensity over the surface of the flow field plate as to form an integralgas diffusion layer.
 10. A flow field plate as claimed in any precedingclaim comprising an electrically conductive core and a non-conductiveframe.
 11. A fuel cell stack comprising a plurality of flow field platesas claimed in claim 9.